Developer Proposes Boundary Independence Principle to Extend SOLID Beyond Components

A software engineer with over a decade of experience writing about SOLID principles has proposed a new design concept called the Boundary Independence Principle (BIP). The author argues that while SOLID effectively guides the writing of clean individual components, it does not address how those components are composed into a system whose physical structure can still evolve. The piece illustrates the gap with a scenario where a well-structured SOLID codebase still requires weeks of rework when business needs demand extracting a service, because system boundaries were baked into import paths and module wiring. The author observes that established architectural patterns — including Hexagonal Architecture, Onion Architecture, and Modular Monoliths — independently converge on the same property: business logic should remain unchanged when the deployment topology shifts. BIP is presented as the underlying principle unifying these patterns, asserting that a system's physical shape should be changeable without modifying the code that performs actual business logic.